The present invention relates to magnetic direction sensing systems and particularly those for use in vehicles.
U.S. Pat. No. 4,953,305, assigned to the present assignee, discloses a magnetic field sensor and microprocessor-controlled compass system for a vehicle. The system senses the magnitude of the earth's magnetic field in two channels of measurement. The sensor data, if plotted on an X-Y coordinate plane, would be as shown in FIG. 1. For a properly calibrated compass, the plot of sensor data creates a perfect circle centered around the origin of the coordinate plane when the vehicle travels in a 360 degree loop, as indicated by graph A of FIG. 1. The radius of the circle represents the detected earth's magnetic field strength, and the vehicle's compass heading at a particular time during travel is represented by a point on the circle. By calculating the angle which the point forms with the X-Y coordinate plane, the compass heading of the vehicle may be determined. As is known, depending on the location of the vehicle, the detected magnitude of the earth's magnetic field can vary significantly.
The sensed magnetic field will also be affected if there is a change in vehicular magnetism. Changes in the magnetism of a vehicle can be caused by, for example, driving the vehicle near the electrical power feeders of train or subway systems, installing a magnetic cellular antennae on the vehicle's roof, parking under an AC powerline, or even driving through a car wash which can flex the sheet metal in the vicinity of the compass sensor and change its magnetic characteristics. Such a change in vehicular magnetism will cause the magnetic field sensed by the compass channels when the vehicle is heading in a given direction to be either greater or lesser than that expected for a vehicle with no magnetic interference. As a result, the plot of sensor data will be shifted away from the origin of the coordinate plane in some direction, resulting in a pattern such as the circle shown as graph B of FIG. 1 when the vehicle travels a 360 degree loop. The magnitude of the shift of sensor data from the origin is proportional to the magnitude of the change in vehicular magnetism.
The compass system of the above-mentioned patent provides automatic and continuous calibration to account for changes in the vehicle's magnetism and thus the system's reaction to the earth's magnetic field during the life of the vehicle. The calibration system includes means for testing the data received from the compass sensor to determine the maximum and minimum signal levels during movement of the vehicle through a completed 360 degree path of travel. This data is averaged over several such paths of vehicular travel to provide continuously updated and averaged compensation correction information. The automatic and continuous calibration is capable of correcting the compass system when the plot of sensor data experiences small shifts away from the origin of the coordinate plane due to small drifts in vehicular magnetism. The origin of the coordinate plane in these circumstances is still contained within the circle plotted when the vehicle travels a 360 degree loop, and the crossings of the sensor data on the axes of the coordinate plane are used to calculate the spans of the signal levels along each axis which determine the center of the circular plot of sensor data. Compensation signals are then generated based on the difference between the center of the circle and the origin of the coordinate plane. However, if the shift of sensor data is large enough such that the origin of the coordinate plane is not contained within the circular plot of sensor data created when the vehicle travels a 360 degree loop, then heading information cannot be calculated and the calibration system cannot provide correction in this somewhat unusual situation unless the sensor data experiences a subsequent shift that causes the origin of the coordinate plane to again be contained. Because such a subsequent shift may never occur or, if it does, may occur only after an undesirably long period of time, the compass system of the above-mentioned patent provides means to reinitiate calibration in these situations.
Reinitiation of calibration involves the collecting and centering of spans of sensor data followed by the collecting and centering of two circles of sensor data which causes the origin of the coordinate plane to coincide with the center of the circular plot of sensor data. As such, the reinitiation process enables the compass system to recover from any change in vehicular magnetism and to provide accurate heading information. In order to detect situations where reinitiation of the calibration system is desirable, it is known to have the compass system maintain saturation limits at the outer boundaries of the range of measurement of the sensor data. For 8-bit sensor data, these saturation limits are at 0 and 255, as shown in FIG. 1. If a large change in vehicular magnetism causes the sensor data to shift and the current data is plotted outside of these limits for a continuous period of five minutes, then calibration is restarted. Such a shift is shown by graph C of FIG. 2, with the dashed portion thereof indicating the range of heading directions of the vehicle that would cause the sensor data to remain outside of the saturation limits. However, intermediate changes in vehicular magnetism are possible which, while causing the plot of sensor data to shift and to not contain the origin of the coordinate plane when the vehicle completes a 360 degree loop, do not cause the sensor data to be plotted outside of the saturation limits. Such a shift is shown by graph D of FIG. 3. As such, it is known to also provide for a reinitiation of calibration if 15 ignition cycles of at least 5 minutes duration are completed without obtaining a crossing point on the axes of the X-Y coordinate plane. Furthermore, it is known to enable the operator of the vehicle to manually reinitiate calibration by operating a switch, button, or the like. Manual reinitiation would most likely occur when the operator notices that the displayed heading information is erroneous for an extended period of time. The above-mentioned means by which to cause reinitiation of calibration enables the compass system to ultimately recover from changes in vehicular magnetism of any magnitude.
Because automatic calibration routines are intended to compensate for the specific vehicular magnetism of the vehicle in which the compass is installed, and because vehicular magnetism can vary greatly from one vehicle to the next, such automatic calibration routines cannot be calibrated before or during installation at the vehicle assembly plant. To acquire sufficient data from the magnetic sensors to have a high level of confidence that the compass system is properly calibrated, automatic calibration routines such as that described above, typically require that the vehicle be driven through at least one 360 degree loop and preferably through two or three 360 degree loops. However, because space constraints at an assembly plant typically do not permit vehicles to be driven in this many loops, newly manufactured vehicles having such compass systems, are typically transported to dealerships before the automatic calibration routine is able to properly calibrate the compass. If these vehicles are not subsequently driven in a sufficient number of loops to calibrate the compass prior to delivery to the buyer, the buyer may be led to believe that the compass system is defective. Due to a large number of warranty claims arising under these circumstances, some automobile manufacturers have now required that suppliers of compass systems ensure that they are precalibrated so as to compensate for any expected vehicular magnetism prior to delivery to the dealerships. The precalibration data used to calibrate the compass would, for example, correspond to average compensation data used for vehicles of a particular make and model.
As noted above, however, vehicular magnetism can vary considerably from vehicle to vehicle even for vehicles of the same make and model. Such variance in magnetism may arise from various options included in these vehicles, such as larger fuel tanks and/or engines. Given this variance in vehicular magnetism, a compass system that is precalibrated correctly for one vehicle may not be properly calibrated for another vehicle of the same model. Thus, absent some mechanism for recalibrating such compass systems after installation, there remains a likelihood that most precalibrated compass systems may not operate properly. Further, when compass systems are calibrated solely using precalibration data, there would be no mechanism in place for correcting any changes in vehicular magnetism that may arise after the vehicle has left the assembly plant.
One approach to solving some of the above-noted problems is to provide an automatically-calibrating compass system that utilizes precalibration data to initially calibrate the compass system until a predetermined event occurs that would initiate the automatic calibration routine. In one such implementation, the compass system initially calculates the vehicle heading utilizing the precalibration data until it is determined that the signal levels output from the sensors have saturated in a manner similar to that discussed above with respect to the automatic calibration routine disclosed in U.S. Pat. No. 4,953,305. Thus, when the compass system determines that 15 ignition cycles of at least 5 minutes duration are completed without obtaining a crossing point on the axes of the X-Y coordinate plane while utilizing the precalibration data, the compass system initiates the automatic calibration routine which then begins accumulating data for purposes of recalibrating the compass system. Another manner of implementing this approach is disclosed in U.S. Pat. No. 5,390,122 issued to Michaels et al. This patent discloses a compass system that initiates an automatic calibration routine for subsequent recalibration of the compass system after the vehicle is first driven at a speed exceeding 10 miles per hour.
One problem with the above approach is that the automatic calibration routine implemented in such compass systems will not have acquired any data needed to properly automatically calibrate the compass at the time at which the automatic compass routine is placed in control of calculating the displayed heading. Thus, it is still possible that the compass system may not be properly calibrated when the vehicle is delivered to the buyer. Additionally, if, prior to delivery to the buyer, the vehicle has not been driven over 10 miles per hour or the compass is inaccurately precalibrated but not yet in a saturated state, the compass system will remain calibrated with the precalibration data at the time of delivery. Subsequently, when the buyer drives the vehicle in excess of 10 miles per hour or the compass becomes saturated, the displayed vehicle heading may abruptly change since the automatic calibration routine has not acquired any data needed for automatic calibration, thereby causing the buyer to believe that the compass system is malfunctioning.
Current vehicle compass systems include some form of visual indication for informing the vehicle occupants of the vehicle heading. The most common form of visual indicator is an electronic display positioned in an overhead console or in a rearview mirror. Such electronic displays are, however, relatively expensive and impose certain constraints on the size and placement of the compass system within the vehicle interior. It would be desirable to provide the operator with an alternate form of compass heading information.